The present invention is related to improvements in lead frames and lead frame bonding wherein the improvements mitigate fatigue cracking in the bond related to thermal cycling. Furthermore in the case of ceramic components such as multilayer ceramic capacitors (MLCC) the present invention mitigates the formation of stress cracks in the ceramic capacitor in addition to the bonding material.
In recent years, chip-type multilayer ceramic capacitors have come into general use for mounting directly to a substrate. However, if the substrate is deformed by deflection, a mechanical stress is transferred to the multilayer ceramic capacitor which may cause a crack to form in the ceramic capacitor. This potential for a crack increases as the size of the capacitor increases due to the increase in the bending moment on the larger capacitor. The deflection is of special concern for capacitors having a principal dimension greater than about 2.54 mm (0.1 inches).
Lead frames have been employed for many years as a connection to one or more capacitors. The lead frames are attached to the external termination of the capacitor and bonded to the substrate by through hole or surface mounting technology to a circuit. A key consideration for the lead frame designs is the ability of the leads to absorb mechanical stresses thereby somewhat isolating the ceramic capacitor, after mounting to a circuit board, from the deflections that occur during temperature cycling. The mechanical stresses are due to differences in the coefficient of thermal expansion (CTE) between the ceramic capacitor and the circuit board. If the lead frame is not effective in absorbing this mismatch cracks form in the ceramic capacitors ultimately leading to electrical failure.
In addition, when ceramic capacitors bonded to metal lead frames are bonded to metal clad substrates possessing high coefficients of expansion, such as aluminum for example, large mechanical stresses can be applied to the ceramic component due to the large difference in the coefficient of thermal expansion of the ceramic component and the substrate thereby causing a stress crack on the ceramic component to occur after only a few temperature cycles.
U.S. Pat. No. 6,081,416 describes the use of nickel iron lead frames with a CTE which is 25% to 50% lower than the CTE of the ceramic of the capacitor. These devices are only suitable for preventing cracking of the ceramic on temperature cycling or thermal shock at temperatures of up to about 150° C.
U.S. Pat. No. 6,515,844 describes a metallic plate of oxygen free copper with slits connected perpendicular to the multilayer ceramic capacitors. The slits can be filled with a substance having a larger coefficient of linear expansion or a substance having a lower Young's modulus with respect to the material comprising the external terminal. More particularly, the filling substance contracts more dramatically than the external terminal at a low temperature, and, accordingly, the deformation (expansion, contraction) of the external terminal is restrained, that is, the expansion and contraction of the external terminal itself are restrained owing to the influence from the expansion and the contraction of the filled substance, and consequently the stress to be applied onto the electronic part is reduced. Moreover, when a substance having a lower Young's modulus, with respect to the external terminal, is provided for filling the slit the stress on the electronic part itself becomes smaller. When exposed to 100 temperature cycles of −55 to +125° C. the presence of multiple slits filled with epoxy, resin or solder were shown to prevent crack formation. This technology is not suitable for use above about 125° C. and certainly not above 150° C.
U.S. Pat. No. 6,310,759 describes a folded lead frame for crack prevention in capacitor stacks wherein one or more holes are provided in the outer portion of the lead to allow the components to be held during assembly. In U.S. Pat. No. 6,181,544 a protrusion from the metal plate to the termination of a multilayer ceramic capacitor is provided for uniformity of bonding material and thermal shock prevention for crack sensitive Pb-based ceramics.
Ceramic capacitors having metal plate terminals that absorb thermal stress are described in U.S. Pat. No. 6,191,933 wherein folded metal plate terminals with at least one hole is provided in the terminal portion. A method of manufacturing these types of capacitor is described in U.S. Pat. No. 6,523,235 using solder paste processing.
The current state of the art is capable of providing lead frame and material designs to lower stresses up to a maximum temperature of about 150° C. However, to facilitate the use of ceramic capacitors at even higher temperatures it has been necessary to use a soft lead attachment to make electrical connection to lead frames connecting two or more ceramic capacitors. This design avoids the mechanical stress caused by constraining the movement of the capacitor on heating in surface mounting or through-hole connections to the electrical circuit. Even with this provision, at temperatures above about 150° C. the lead frame must be bonded to the ceramic capacitors using a bonding material designed to avoid excessive stress due to the propensity for cracking of the bond between the lead frame and the capacitor.
In spite of the ongoing efforts there is still no suitable solution for larger capacitors, especially for use in high temperature environments such as above 150° C. A capacitor suitable for such uses is provided herein.